This invention relates to a process for the smelting of silicon dioxide and silicon carbide to produce silicon.
At present, silicon is typically produced in a submerged electric arc furnace via the carbothermic reduction of silicon dioxide (SiO.sub.2) with a solid carbonaceous reducing agent such as coke, coal, or wood chips. The overall reduction reaction can be represented as, EQU SiO.sub.2 +2C =Si+2CO
It is generally recognized that the above reaction in reality involves multiple reactions, the most significant being outlined below: EQU SiO.sub.2 +3C=SiC+2CO (1), EQU SiO.sub.2 +C=SiO+CO (2), EQU SiO+2C=SiC+CO (3), EQU 2SiO.sub.2 +SiC=3SiO+CO (4), EQU and EQU SiO+SiC=2Si+CO (5).
Muller et al., Scand. J. Metall., 1 (1972), pp. 145-155, describes and defines the theoretical equilibrium conditions for the Si--O--C chemical system of the carbothermic reduction of silicon dioxide to form silicon. A critical teaching of Muller et al. is the limitation that under equilibrium conditions the partial pressure of silicon monoxide (SiO) must be equal to or greater than 0.67 atmospheres at atmospheric pressure and at a temperature of 1819.degree. C. for reaction (5), above, to occur to form silicon. By-produced carbon monoxide (CO) can have an inhibiting effect upon the formation of molten silicon.
The use of a submerged electric arc furnace for the production of silicon has been used on a commercial basis for many years. More recently the use of a transferred arc plasma as an energy source for the carbothermic reduction of silicon dioxide has been utilized.
Several references cite the use of SiC as a feed to a silicon furnace. These references include: Potter, U.S. No. 875,672, issued Dec. 31, 1907; Lask, U.S. No. 4,366,137, issued Dec. 28, 1982; British Patent 18,659, published July 28, 1900; UK Patent Application GB 2,150,128A, published June 26, 1985; Russian Patent 599,561, published May, 1958; Japanese Patent Publication 49-37686, published Oct. 11, 1974; European Patent Application 0 177 894, published Apr. 27, 1988.
Enger et al. U.S. No. 3,887,359, issued June 3, 1975, discloses the feeding of SiO.sub.2 and carbon separately in separate zones of an electric arc furnace wherein the reaction gases pass through one or more zones rich in carbon. This invention is an attempt to minimize the losses of SiO.
Johannson, U.S. No. 4,269,620, issued May 26, 1981, discloses a continuous silicon smelting process for producing silicon from the reaction of SiO with SiC in the energy zone of a furnace. In the process disclosed by Johannson SiO.sub.2 (or a mixture of SiO.sub.2 and carbon) and carbon are fed countercurrently toward the energy zone of the furnace. There are no apparent references to provisions for mixing the SiO.sub.2 fed and the generated SiC. In the absence of mixing, one is led to conclude that control of the proportion of reactants within the furnace may not be adequate to effect efficient silicon production.
None of these references disclose a cyclic, two-step batch procedure in which SiO.sub.2 and SiC are reacted to form molten silicon, SiO, and CO, the SiO then being contacted with a bed of carbon to regenerate SiC.